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PubMed1996

Oxo-Centered Mixed-Ligand Triruthenium Complexes Having Redox-Active N-Methyl-4,4'-bipyridinium Ions (mbpy(+)). Reversible Multistep Electrochemical Properties of [Ru(III)(2)Ru(II)(&mgr;(3)-O)(&mgr;-CH(3)CO(2))(6)(mbpy(+))(2)(CO)](2+) and [Ru(III)(3)(&mgr;(3)-O)(&mgr;-CH(3)CO(2))(6)(mbpy(+))(2)(L)](3+) (L = H(2)O and N-Heterocyclic Ligands).

A new series of oxo-centered acetate-bridged triruthenium comlexes having two redox-active N-methyl-4,4'-bipyridinium ions (mbpy(+)) have been prepared, and their reversible multistep and multielectron electrochemical properties are reported: [Ru(III)(2)Ru(II)(&mgr;(3)-O)(&mgr;-CH(3)CO(2))(6)(mbpy(+))(2)(CO)](2+) and [Ru(III)(3)(&mgr;(3)-O)(&mgr;-CH(3)CO(2))(6)(mbpy(+))(2)(L)](3+) (L = H(2)O, pyrazine (pz), pyridine (py), imidazole (Him), and 4-(dimethylamino)pyridine (dmap)). Among these series, the CO complex, [Ru(III)(2)Ru(II)(&mgr;(3)-O)(&mgr;-CH(3)CO(2))(6)(mbpy(+))(2)(CO)](ClO(4))(2).2DMF (1b.2DMF) was structurally characterized by X-ray crystallography. 1b.2DMF crystallizes in the monoclinic space group P2(1)/m (No. 11) with a = 8.740(6) Å, b = 32.269(6) Å, c = 10.276(4) Å, beta = 103.37(5) degrees, V = 2820(2) Å(3), Z = 2, d(calcd) = 1.636 g cm(-)(3), and R = 0.071 (R(w) = 0.074) for 5277 independent reflections (|F(o)| > 3sigma(|F(o)|). The (CO)Ru.Ru distance (3.410(2) Å) is appreciably longer than the other Ru.Ru distance (3.276(2) Å), indicating that the trinuclear core is in the valence-trapped Ru(III)(2)Ru(II)(CO) oxidation state. The cyclic voltammogram of [Ru(III)(2)Ru(II)(&mgr;(3)-O)(&mgr;-CH(3)CO(2))(6)(mbpy(+))(2)(CO)](PF(6))(2) (1a) shows a total of seven reversible one-electron redox steps at E(1/2) = +0.90, +0.26, -1.07, -1.17, -1.56, -1.97, and -2.32 V and one irreversible step at E(pc) = -2.99 V vs Fc/Fc(+) in a 0.1 M [(n-C(4)H(9))(4)N]PF(6)-CH(3)CN solution (M = mol dm(-)(3)). All of the waves are clearly assignable to the triruthenium "Ru(3)(&mgr;(3)-O)" core-based or mbpy(+) ligand-based processes. The splitting of each ligand-based redox processes (mbpy(+)/mbpy(*) and mbpy(*)/mbpy(-)) into two one-electron steps indicates that electronic interactions between two terminal ligands occur through the triruthenium cluster core. Other mixed-ligand Ru(III)(3) analogs also show multistep redox behavior involving a total of eight or nine electrons. While the extent of interactions between ligands is much smaller than that found in the CO complex, it is systematically changed by the nature of L; with more basic L, interactions between two mbpy(+) ligands become larger.

Journal Article↗
PubMed1997

Paramagnetism of Tetranuclear Complexes between TCNX Ligands (TCNE, TCNQ, TCNB) and Four Pentaammineruthenium or Dicarbonyl(pentamethylcyclopentadienyl)manganese Fragments.

The tetranuclear complexes {(&mgr;(4)-TCNX)[Ru(NH(3))(5)](4)}(A)(8) and (&mgr;(4)-TCNX)[Mn(CO)(2)(C(5)Me(5))](4) [A = PF(6) or CF(3)SO(3); TCNX = TCNE (tetracyanoethene), TCNQ (7,7,8,8-tetracyano-p-quinodimethane), or TCNB (1,2,4,5-tetracyanobenzene)] were studied by variable-temperature (2-300 K) SQUID susceptometry. Mono- and dinuclear species [(PhCN)Ru(NH(3))(5)](PF(6))(2) (PhCN = benzonitrile) and {(&mgr;-L)[Ru(NH(3))(5)](2)}(PF(6))(4) (L = 1,4-dicyanobenzene (terephthalodinitrile) or pyrazine) were also investigated for comparison and were found to be essentially diamagnetic. Despite the even electron count, both the ruthenium and manganese tetranuclear complexes are paramagnetic, albeit with different spin-spin exchange coupling patterns. The manganese systems are characterized by exchange-coupled S = 1 states at the individual metal centers, whereas the magnetic behavior of the tetranuclear ruthenium compounds results from an exchange-coupling interaction between two S = (1)/(2) sites, identified as Ru(III)/Ru(II) mixed-valence pairs.

Journal Article↗
PubMed1997

Electrochemical, Spectroscopic, and Spectroelectrochemical Properties of Synthetically Useful Supramolecular Light Absorbers with Mixed Polyazine Bridging Ligands.

The trimetallic complexes [{(bpy)(2)M(dpp)}(2)Ru(dpq)](6+) (M = Ru(II) or Os(II), bpy = 2,2'-bipyridine, dpp = 2,3-bis(2-pyridyl)pyrazine, and dpq = 2,3-bis(2-pyridyl)quinoxaline) have been prepared and the details of their spectroscopic, electrochemical, and spectroelectrochemical properties investigated. These mixed bridging ligand complexes are a new group of synthons that can be useful for the construction of supramolecular devices for a wide variety of functions. It is the presence of the terminal dpq ligand that allows for their incorporation into larger supramolecular systems. This dpq ligand serves as an acceptor ligand that will possess a lower lying pi orbital than the dpp ligands once these chromophores are incorporated into larger systems. The [{(bpy)(2)M(dpp)}(2)Ru(dpq)](6+) systems display overlapping terminal metal oxidations at 1.66 and 1.18 V vs Ag/AgCl for the Ru and Os systems, respectively. This indicates that within this framework, these terminal, M, metals are largely electronically uncoupled. No oxidative process for the central Ru metal center is observed within our solvent window. The [{(bpy)(2)M(dpp)}(2)Ru(dpq)](6+) systems have M --> dpp charge transfer (CT) lowest lying excited states. The [{(bpy)(2)Ru(dpp)}(2)Ru(dpq)](6+) Ru --> dpp CT state displays an emission centered at 775 nm with a lifetime of 65 ns at room temperature in deoxygenated CH(3)CN solution. The details of the electrochemical, spectroscopic, and spectroelectrochemical studies of these supramolecular light absorbers and the dichloro synthons, [{(bpy)(2)M(dpp)}(2)RuCl(2)](4+), are reported herein.

Journal Article↗
PubMed1998

Synthesis and Spectral Properties of Bisaxially Coordinated (Octaphenyltetraazaporphyrinato)ruthenium(II) Complexes.

The synthesis of novel bisaxially coordinated ruthenium(II) complexes of octaphenyltetraazaporphine (L(2)Ru(OPTAP); L = NH(3), pyridine (py), pyrazine (pyz), s-tetrazine (tz), tert-butyl isocyanide (t-BuNC), and p-diisocyanodurole (did)) is reported. Template condensation of diphenylfumaronitrile with pentaammine(dinitrogen)ruthenium(II) dichloride gives the crude compound "Ru(OPTAP)" containing mainly the bis(ammine) derivative (NH(3))(2)Ru(OPTAP). Heating of "Ru(OPTAP)" with excess of L (L = py, pyz, tz, t-BuNC, did) gives L(2)Ru(OPTAP) which are readily soluble in organic solvents and can be purified chromatographically. In the case of the bidentate ligands (L = pyz, tz, did) refluxing of the solutions of monomers L(2)Ru(OPTAP) results in formation of oligomers with the bridged &mgr;-L coordination. For L = pyz and tz, the sparingly soluble oligomers contain predominantly dimers [(L){Ru(OPTAP)}(&mgr;-L){Ru(OPTAP)}(L)], and, for L = did, the insoluble &mgr;-bridged polymer [(did){Ru(OPTAP)}((&mgr;-did){Ru(OPTAP)})(x)()(>10)(did)] is formed. UV-visible, IR, and (1)H NMR spectroscopy and mass spectrometry were used for characterization of the obtained compounds. Cyclic voltammetry study has shown one reduction (OPTAP(2)(-)/OPTAP(3)(-)) and one oxidation (Ru(II)/Ru(III)) process (for (py)(2)Ru(OPTAP) observed at -1.15 and 0.68 V, respectively). Comparison with the data on the corresponding phthalocyanine complexes (L(2)Ru(Pc)) reveals the stronger pi-back-bonding properties, the higher oxidation potential, and lower aromaticity of the porphyrazine macrocycle present in L(2)Ru(OPTAP). These factors along with the lesser extent of oligomerization are responsible for the lower electrical conductivity (sigma(RT) < 10(-)(11) S/cm) of the &mgr;-bridged Ru(OPTAP) complexes as compared with that of the &mgr;-bridged RuPc polymers.

Journal Article↗
PubMed1998

A Tridentate-Bridged Ruthenium-Rhodium Complex as a Stereochemically Defined Light-Absorber-Electron-Acceptor Dyad.

The complex [(tpy)Ru(tpp)RhCl(3)](PF(6))(2) (tpy = 2,2',6',2"-terpyridine and tpp = 2,3,5,6-tetrakis(2-pyridyl)pyrazine) has been prepared and its spectroscopic, electrochemical, and photophysical properties investigated. This complex couples a ruthenium light absorber to a rhodium electron acceptor to create the first tpp-bridged light-absorber-electron-acceptor dyad. This study illustrates the applicability of this (tpy)Ru(II)(&mgr;-tpp) chromophore in the construction of photochemical molecular devices. This system is of interest since the tpp ligand has been shown to provide stereochemically defined polymetallic complexes with reasonably long-lived metal to ligand charge transfer excited states. The complex [(tpy)Ru(tpp)RhCl(3)](PF(6))(2) displays a Ru-->tpp CT transition centered at 516 nm that is the lowest lying electronic transition. The electrochemistry of [(tpy)Ru(tpp)RhCl(3)](PF(6))(2) shows a Ru(II/III) couple at 1.60 V vs Ag/AgCl, an irreversible Rh(III/I) reduction at -0.23 V and, a tpp(0/)(-) couple at -0.60 V. This illustrates that although this complex has a lowest lying spin-allowed spectroscopic transition that is Ru-->tpp CT in nature, the lowest occupied molecular orbital is Rh based. Thus, following excitation of this [(tpy)Ru(tpp)RhCl(3)](PF(6))(2) complex into the Ru-->tpp CT state, electron transfer to the rhodium is thermodyamically favorable. This electron transfer leads to a quenching of the emission normally observed for this Ru-->tpp CT excited state. Emission quenching for [(tpy)Ru(tpp)RhCl(3)](PF(6))(2) via electron transfer is 80% efficient with a k(et) of 4 x 10(7) s(-)(1). Details of these studies are presented herein.

Journal Article↗
PubMed1998

Neutral Guest Capture via Lewis Acid/Base Molecular Square Receptors. X-ray Crystal Structure of {Cyclobis[(cis-(dppp)Pt(4-ethynylpyridyl)(2))(cis- (PEt(3))(2)Pt)]Ag(2)}(+6)(phenazine)- ((-)OSO(2)CF(3))(6).

Interaction of {cyclobis[(cis-(dppp)Pt(4-ethynylpyridyl)(2))(cis-(L)M)]Ag(2)}(+6)((-)OSO(2)CF(3))(6), where M = Pt(II) or Pd(II) and L = dppp or 2PEt(3), with pyridine, pyrazine, phenazine, or 4,4'-dipyridyl ketone results in coordination Lewis acid/base host-guest assemblies via the "pi-tweezer effect" and mono or bis neutral guest coordination. All host-guest complexes are air stable microcrystalline solids with decomposition points greater than 170 degrees C. The homometallic Pt(II) receptors are more stable than the heteroaromatic Pt(II)-Pd(II) receptors toward heteratom-containing aromatic guests. The X-ray crystal structure of the host-guest complex {cyclobis[(cis-(dppp)Pt(4-ethynylpyridyl)(2))(cis-(PEt(3))(2)Pt)]Ag(2)}(+6)(phenazine)((-)OSO(2)CF(3))(6) is reported. The crystals with the empirical formula C(62)H(68)AgF(9)N(3)O(9)P(4)Pt(2)S(3) are triclinic P&onemacr; with a = 12.3919(8) Å, b = 17.160(1) Å, c = 18.932(1) Å, alpha = 90.892(1) degrees, beta = 97.127(1) degrees, gamma = 89.969(1) degrees, and Z = 2.

Journal Article↗
PubMed1999

Synthesis and Spectroscopic Characterization of a Zeolite-Entrapped Ru(bpy)(2)(dpp)(2+) Complex.

The heteroleptic ruthenium(II) complex Ru(bpy)(2)(dpp)(2+) (where bpy = 2,2'-bipyridine and dpp = 2,3-bis(2-pyridyl)pyrazine) has been prepared in the supercages of Y-zeolite and characterized by diffuse reflectance, electronic absorption, electronic emission, and resonance Raman (RR) spectroscopy, as well as lifetime measurements. The spectral results confirm the identity of the entrapped complex. The diffuse reflectance spectrum of the zeolite-entrapped complex is slightly red-shifted in the visible region compared to the absorption spectrum of the free complex in water solution. The emission maximum for the zeolite-entrapped complex is red-shifted by 16 nm with respect to the complex in water solution and by 38 nm relative to the complex in acetonitrile solution. The emission maximum for a zeolite surface-adsorbed complex is blue-shifted by 12 nm with respect to the complex in water solution, but the emission maxima of the liberated complex after dissolution of the zeolite matrix, for both the entrapped and adsorbed complexes, are identical to the spectrum of the free complex in water solution. There are moderate RR frequency shifts observed for the zeolite-entrapped complex relative to the solution-phase complex as a consequence of the interaction of the complex with the zeolite framework. The excited state lifetime measurement shows no dramatic changes upon entrapment within the zeolite supercages. Temperature-dependent lifetime measurements indicate that the excited state decays via two thermally accessible upper states.

Journal Article↗
PubMed2000

Protonation studies of reduced ruthenium(II) complexes with polypyridyl ligands.

The pKa values associated with protonation of the one-electron reduced forms of series of [L'2Ru(II)L]2+ complexes [L' = bidentate polypyridyl ligand; L = bidentate polypyridyl ligand with additional uncoordinated N atoms in the aromatic ring system: e.g., dpp = 2,3-bis(2-pyridyl)pyrazine, bpz = 2,2'-bipyrazine] were assessed using pulse radiolysis techniques by the measurement of spectral variations as a function of pH. A linear correlation was observed between pKa and E (RuL'2L2+/+) for complexes in which the protonatable ligand was at the same time the site of reduction. In complexes where one or more of the nonprotonatable ligands (L') had very low pi* energy levels [e.g. (CF3)4bpy], reduction occurs on a nonprotonatable ligand and a dramatic decrease in the pKa values was observed for the reduced species. In complexes where the energies of the protonatable and nonprotonatable ligands were comparable, the protonation behavior was consistent with some orbital mixing/ delocalization of the electronic charge.

Journal Article↗
PubMed2000

Ruthenium bisbipyridine complexes of horse heart cytochrome c: characterization and comparative intramolecular electron-transfer rates determined by pulse radiolysis and flash photolysis.

The reaction of [Ru(bpy)2L(H2O)]2+ (bpy = 2,2'-bipyridine, L = imidazole, water) with reduced horse heart cytochrome c results in coordination of [RuII(bpy)2L] at the His 33 and His 26 sites. Coordination at the His 33 site gave a diastereomeric [RuII(bpy)2L]-His-cyt c(II) mixture favoring the lambda-Ru form regardless of the substituent on the bipyridine ligands, while substitution at the more buried His 26 site gave an isomeric distribution that varies according to the substituent on the bipyridine ligands. The diastereomeric aquoproteins (L = H2O) are distinguished by their redox potentials and their conversion to the corresponding fluorescent imidazole proteins. Intramolecular electron transfer between the reduced ruthenium bipyridine and cyt c(III) in [RuII(bpy.)(bpy)L]-His33-cyt c(III) was determined by reductive pulse radiolysis using the aqueous electron as a reducing agent, kret = (2.0 +/- 0.3) x 10(5) s-1, and kret is independent of the sixth ligand L = H2O, imidazole. In addition, the rate constant for intramolecular electron transfer from cyt c(II) to the ruthenium(III) center in [RuIII(bpy)2L]-His33-cyt c(II) was determined by oxidative pulse radiolysis using azide and carbonate radicals. This rate is very sensitive to the nature of the sixth ligand. When L = H2O, the intramolecular electron-transfer rate for the major diastereomer lambda-cis-[RuIII (bpy)2(H2O)]-His33-cyt c(II) is k = 1.1 x 10(4) s-1 and is independent of pH between 5.6 and 8.3. The minor delta-cis-[RuIII(bpy)2(H2O)]-His33-cyt c(II) isomer has pH-dependent electrochemistry and a lower rate of intramolecular electron transfer. Complete conversion from L = H2O to L = imidazole is slow, requiring more than 7 days in 1 M imidazole. A lower limit (k > 2 x 10(6) s-1) for the intramolecular electron-transfer rate constant in [RuIII(bpy)2(L)]-His33-cyt c(II), L = imidazole, could be obtained by pulse radiolysis in the absence of the slower reacting aquo species. This observation is in agreement with the value of 3 x 10(6) s-1 measured by flash photolysis. Earlier pulse radiolysis experiments primarily measured the aquoligated ruthenium protein, while the flash photolysis experiments measured the imidazole-ligated fraction because it is the only species oxidatively quenched in the photoinduced reactions. Intramolecular electron-transfer reactions for a new series of ruthenium bipyridine complexes, [Ru(dabpy)2L]-His33-cyt c proteins (dabpy = 4,4'-diamino-2,2'-bipyridine) (L = imidazole, pyridine, isonicotinamide and pyrazine), proceed with lower driving force, resulting in slower rate constants amenable to measurement by oxidative pulse radiolysis. The electron-transfer rate constants for this series spanned a wide range of the Marcus log k vs delta G plot.

Animals↗
PubMed2002

Direct measurement of excited-state intervalence transfer in [(tpy)Ru(III)(tppz(*-))Ru(II)(tpy)](4+) by time-resolved near-infrared spectroscopy.

Extension of time-resolved infrared (TRIR) measurements into the near-infrared region has allowed the first direct measurement of a mixed-valence band in the metal-to-ligand charge transfer (MLCT) excited state of a symmetrical ligand-bridged complex. Visible laser flash excitation of [(tpy)Ru(tppz)Ru(tpy)]4+ (tppz is 2,3,5,6-tetrakis(2-pyridyl)pyrazine; tpy is 2,2':6',6' '-terpyridine) produces the mixed-valence, MLCT excited state [(tpy)RuIII(tppz*-)RuII(tpy)]4+* with the excited electron localized on the bridging tppz ligand. A mixed-valence band appears at numax = 6300 cm-1 with a bandwidth-at-half- maximum, Deltanu1/2 = 1070 cm-1. In the analogous ground-state complex, [(tpy)Ru(tppz)Ru(tpy)]5+, a mixed-valence band appears at numax = 6550 cm-1 with Deltanu1/2 = 970 cm-1 which allows a comparison to be made of electronic coupling across tppz0 and tppz*- as bridging ligands.

Journal Article↗
PubMed2002

Light-harvesting heptadecameric porphyrin assemblies.

New porphyrin assemblies containing 17 porphyrin molecules are constructed by using free base TPP-type porphyrins having eight pyrazine moieties 1. Spectroscopic titration of dimeric [meso-tetrakis(2-carboxy-4-nonylphenyl)porphyrinato]zinc(II) 2 with these porphyrins shows that the processes of the formation of the heptadecameric porphyrin assemblies may be analyzed as eight independent equilibrium processes with an identical binding constant. All binding constants are larger than 5 x 107 M-1 which is the determinable upper limit of the present titration method. In all cases, the fluorescence spectrum of the 1:8 mixture of 1 and 2 consists of the major fluorescence of 1 and the minor one of 2.pyrazine complex even in the presence of the large excess of the antenna pigments. The observed spectra are well reconstructed by the form of faF1 + fbF2, where F1 and F2 are the fluorescence of 1 and the 2.pyrazine complex measured separately at the corresponding concentrations. Interestingly, the general trend that values of fa are nearly equal to those of r564 x (1 - fb) in all cases is found, where r564 is the absorption ratios of the 2.pyrazine moiety and the central free base porphyrin in the assemblies at 564 nm. The observation indicates the excitation of the central porphyrin is directly enhanced by the absorption of the antenna pigments even in such large scale assemblies. Thus, the antenna effect for 1 having largest r564 results in 77 times fluorescence enhancement of the central free base porphyrin. The systems also show interesting dependency of energy-transfer efficiencies on the topological arrangement of the antenna elements.

Metalloporphyrins↗
PubMed2002

Solvent dynamical control of electron-transfer rates in mixed-valence complexes observed by infrared spectral line shape coalescence.

Rate constants for intramolecular electron transfer within the intervalence charge transfer (-1) states of the complexes [{Ru3O(OAc)6(L)(CO)}2(mu-pz)] (where L= 4-(dimethylamino)pyridine (1), pyridine (2), 3-cyanopyridine (3), or 4-cyanopyridine (4) and pz = pyrazine) were determined by coalescence of infrared (IR) vibrational spectral line shapes in seven solvents. The electron-transfer times (kET-1) show a strong correlation with solvent relaxation times determined in separate ultrafast time-resolved fluorescence experiments. The best comparison is found with the parameter t1e, which is ascribed to inertial solvent relaxation. The IR spectra of these mixed-valence complexes are thus a steady-state spectral probe of ultrafast, dynamic solvent relaxation processes which are otherwise only accessible using laser-pumped, ultrafast time-resolved measurements.

Journal Article↗
PubMed2003

X- and W-band EPR and Q-band ENDOR studies of the flavin radical in the Na+ -translocating NADH:quinone oxidoreductase from Vibrio cholerae.

Na(+)-NQR is the entry point for electrons into the respiratory chain of Vibrio cholerae. It oxidizes NADH, reduces ubiquinone, and uses the free energy of this redox reaction to translocate sodium across the cell membrane. The enzyme is a membrane complex of six subunits that accommodates a 2Fe-2S center and several flavins. Both the oxidized and reduced forms of Na(+)-NQR exhibit a radical EPR signal. Here, we present EPR and ENDOR data that demonstrate that, in both forms of the enzyme, the radical is a flavin semiquinone. In the oxidized enzyme, the radical is a neutral flavin, but in the reduced enzyme the radical is an anionic flavin, where N(5) is deprotonated. By combining results of ENDOR and multifrequency continuous wave EPR, we have made an essentially complete determination of the g-matrix and all major nitrogen and proton hyperfine matrices. From careful analysis of the W-band data, the full g-matrix of a flavin radical has been determined. For the neutral radical, the g-matrix has significant rhombic character, but this is significantly decreased in the anionic radical. The out-of-plane component of the g-matrix and the nitrogen hyperfine matrices are found to be noncoincident as a result of puckering of the pyrazine ring. Two possible assignments of the radical signals are considered. The neutral and anionic forms of the radical may each arise from a different flavin cofactor, one of which is converted from semiquinone to flavohydroquinone, while the other goes from flavoquinone to semiquinone, at almost exactly the same redox potential, during reduction of the enzyme. Alternatively, both forms of the radical signal may arise from a single, extremely stable, flavin semiquinone, which becomes deprotonated upon reduction of the enzyme.

Bacterial Proteins↗
PubMed2002

[(Cyclen)(4)Ru(4)(pz)(4)](9+): a Creutz-Taube square.

The use of cyclen (1,4,7,10-tetraazacyclododecane) as a blocking ligand enables assembly of the mixed-valence square complex [(cyclen)4Ru4(pz)4]9+ (pz = pyrazine). A crystal structure determination shows the molecule to possess a regular square geometry wherein each Ru atom has an equivalent coordination environment. Consistent with the presence of one RuIII and three RuII centers, cyclic voltammetry reveals a single reversible reduction wave and three successive oxidation waves. The separation between the first oxidation and reduction waves indicates a comproportionation constant of Kc = 108.9 for the [(cyclen)4Ru6(pz)4]9+ square, suggesting a greater extent of electron delocalization than that observed for the Creutz-Taube ion. The closer spacing between oxidation waves suggests a lesser degree of delocalization in the [(cyclen)4Ru6(pz)4]10+ (Kc = 102.0) and [(cyclen)4Ru6(pz)4]11+ (Kc = 103.0) species, bearing the higher average oxidation states of Ru2.5+ and Ru2.75+, respectively.

Journal Article↗
PubMed2003

Ultrafast processes in bimetallic dyads with extended aromatic bridges. Energy and electron transfer pathways in tetrapyridophenazine-bridged complexes.

The energy and electron transfer processes taking place in binuclear polypyridine complexes of ruthenium and osmium based on the tetrapyrido[3,2-a:2',3'-c:3' ',2' '-h:2' "-3' "-j]phenazine bridging ligand (tpphz) have been investigated by ultrafast absorption spectroscopy. In the binuclear complexes, each chromophore is characterized by two spectrally distinguishable metal-to-ligand charge transfer (MLCT) excited states: MLCT1 (with promoted electron mainly localized on the bpy-like portion of tpphz, higher energy) and MLCT0 (with promoted electron mainly localized on the pyrazine-like portion of tpphz, lower energy). In the homodinuclear complexes Ru(II)-Ru(II) and Os(II)-Os(II), MLCT1 --> MLCT0 relaxation (intraligand electron transfer) is observed, with strongly solvent-dependent kinetics (ca. 10(-10) s in CH2Cl2, ca. 10(-12) s in CH3CN). In the heterodinuclear Ru(II)-Os(II) complex, *Ru(II)-Os(II) --> Ru(II)-Os(II) energy transfer takes place by two different sequences of time-resolved processes, depending on the solvent: (a) in CH2Cl2, ruthenium-to-osmium energy transfer at the MLCT1 level followed by MLCT1 --> MLCT0 relaxation in the osmium chromophore, (b) in CH3CN, MLCT1 --> MLCT0 relaxation in the ruthenium chromophore followed by osmium-to-ruthenium metal-to-metal electron transfer. In the mixed-valence Ru(II)-Os(III) species, the *Ru(II)-Os(III) --> Ru(III)-Os(II) electron transfer quenching is found to proceed by two consecutive steps in CH3CN: intraligand electron transfer followed by ligand-to-metal electron transfer. On a longer time scale, charge recombination leads back to the ground state. Altogether, the results show that the tpphz bridge plays an active mechanistic role in these systems, efficiently mediating the transfer processes with its electronic levels.

Journal Article↗
PubMed2003

Cyclic polyamidato dianions as bridges between Mo(2)(4+) units: synthesis, crystal structures, electrochemistry, absorption spectra, and electronic structures.

Compounds in which quadruply bonded Mo(2)(4+) units, Mo(2)(DAniF)(3) (DAniF = N,N'-di-p-anisylformamidinate), are linked by cyclic diamidate anions have been synthesized and characterized by X-ray crystallography and spectroscopic methods. As identified by the diamidate linker, these compounds are 4,6-dioxypyrimidinate (2), 2,3-dioxypyrazinate (3), 2,3-dioxyquinoxalinate (4), 2,3-dioxy-5,6-dicyanopyrazinate (5), and cyanurate (6). With uracilate, a dinuclear unlinked 1:1 adduct is formed, Mo(2)(DAniF)(3)(uracilate) (1). The cyclic voltammograms of 3-5 reveal significantly larger DeltaE(1/2) values (258 mV-308 mV) than that of the oxalate linked analogue (212 mV), which is indicative of greater charge delocalization in the mixed valent Mo(2)(4+)/Mo(2)(5+) species and hence greater communication between the two Mo(2) units. DeltaE(1/2) for 2 is substantially lower than those for 3-5. This difference is attributed to the meta disposition of the two amidate groups in 4,6-dioxypyrimidinate as compared to their ortho arrangement in the pyrazinate-type linkers. The absorption spectra of the linked compounds 3-5 are more complex than those of the analogous polyunsaturated dicarboxylate linked compounds and reveal at least two significant absorption bands within the region 420-550 nm. Compound 2 also has two bands but with significantly lower intensity. Time dependent DFT calculations upon 2 and 3 indicate rather different electronic structures for these two structural isomers. The two bands for 3 have delta --> pi character, and the pi type orbitals have substantial contributions from the Mo(2) units as well as from the diamidate linker. The excitations observed in 2 are mainly metal based. The differences between the electronic spectra of 2 and 3 are consistent with the electrochemistry in underscoring the profound physical effect of changing the symmetry of the diamidate linker.

Anions↗
PubMed2003

Intervalence involvement of bridging ligand vibrations in hexaruthenium mixed-valence clusters probed by resonance Raman spectroscopy.

Resonance Raman spectroscopy, performed using spectroelectrochemistry and with excitation in the intervalence bands of three pyrazine-bridged, mixed-valence dimers of trinuclear ruthenium clusters, shows resonant enhancement of symmetric bridging ligand modes. The resonant enhancements and frequency shifts of these bridging ligand modes are observed as a function of varying electronic communication between charge sites, and they show that a three-state vibronic model which explicitly includes the participation of the bridging ligand is needed to explain the spectroscopic behavior of these near-delocalized complexes.

Journal Article↗
PubMed2003

Methyl side-chain dynamics in proteins using selective enrichment with a single isotopomer.

13C relaxation studies on side-chain methyl groups in proteins typically involve measurements on (13)CHD(2) isotopomers, where the (13)C relaxation mechanism is particularly straightforward in the presence of a single proton. While such isotopomers can be obtained in proteins overexpressed in bacteria by use of (13)C enriched and fractionally deuterated media, invariably all possible (2)H isotopomers are obtained. This results in a loss of both resolution and sensitivity, which becomes particularly severe for larger proteins. We describe an approach that overcomes this problem by chemical synthesis of amino acids containing a pure (13)CHD(2) isotopomer. We illustrate the benefits of this approach in (13)C side-chain relaxation measurements on the mouse major urinary protein selectively enriched with [gamma(1),gamma(2)-(13)C(2),alpha,beta,gamma(1),gamma(1),gamma(2),gamma(2)-(2)H(6)] valine. Relaxation measurements in the absence and presence of pyrazine-derived ligands suggest that valine side-chain dynamics do not contribute significantly to binding entropy.

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